Record of Early Toarcian carbon cycle perturbations in a nearshore environment: the Bascharage section (easternmost Paris Basin)

F Baudin,M Hermoso,L Le Callonnec,M Renard,F Minoletti,A Faber,D Delsate

doi:10.5194/se-5-793-2014

Abstract

Abstract. In order to understand the significance of the worldwide deposition of black shale facies in the Early Toarcian (~ 183 Ma), considerable attention has been drawn to this Early Jurassic sub-stage over the last 3 decades. The discovery of a pronounced negative carbon isotope excursion (CIE) within the black shales disrupting the generally positive trend in carbon isotopes has stimulated many studies, particularly with a view to establish the local versus global nature of this major geochemical phenomenon. Here we document the sedimentological and chemostratigraphic evolution of a proximal environment in the Luxembourgian sedimentary area. At Bascharage, Lower Toarcian sediments record the isotopic signature of the Early Toarcian oceanic anoxic event (OAE) by a pronounced positive trend that testifies for widespread anoxia. The expression of the carbon isotope perturbation in this section, however, is unusual compared to adjacent NW European sections. A first −7‰ negative CIE, whose onset is recorded at the top of the tenuicostatum zone, can be assigned to the well-documented and potentially global Toarcian carbon isotope excursion (T-CIE) with confidence using the well-constrained ammonite biostratigraphic framework for this section. In this interval, facies contain only a limited amount of carbonate as a result of intense detrital supply in such a proximal and shallow environment. Stratigraphically higher in the section, the serpentinum zone records a subsequent CIE (−6‰) expressed as four negative steps, each being accompanied by positive shifts in the oxygen isotopic composition of carbonate. The preservation state of coccoliths and calcareous dinoflagellates in the second CIE is excellent and comparable to that observed in under- and overlying strata, so this cannot be an artefact of diagenesis. Considering the nature of this record, and the lack of such a pronounced event in the serpentinum zone in coeval sections in Europe, we hypothesise that this second CIE was caused by local factors. The geochemical record of carbonate with a relatively light carbon and relatively heavy oxygen isotopic composition is compatible with the so-called Küspert model, by which a CIE can be explained by an influx of 12C-rich and cold waters due to upwelling bottom water masses.

Highlights

Despite multiple studies that have attempted characterisation of the evolution in palaeoenvironment coeval with the accumulation of black shales in the Early Toarcian, we still need to gain a mechanistic understanding of the combination of biological and oceanographic processes that drove seawater into an anoxic/euxinic state
It has to be noted that even if the lithological expression of this limestone level is clear with respect to the hosting black shale facies, the increase in carbonate is elevated in both under- and overlaying sediments
Deposition of black shales in the Early Toarcian interval overlies a transitional facies consisting of grey marls with abundant marine fauna indicating re-establishment of full marine conditions

Summary

Introduction

Despite multiple studies that have attempted characterisation of the evolution in palaeoenvironment coeval with the accumulation of black shales in the Early Toarcian, we still need to gain a mechanistic understanding of the combination of biological and oceanographic processes that drove seawater into an anoxic/euxinic state. The whole Early Jurassic epoch recorded evidence for oxygen restriction in the European epicontinental seaways, but some intervals corresponded to more severe episodes of oxygen depletion, as during the Early Toarcian oceanic anoxic event – commonly referred to as the T-OAE (Jenkyns, 1988). As most of the documented sections hitherto originate from the former European epicontinental seaways, it is essential to untangle the influence of local settings (shallow water depth, short distance from the coast, restricted marine circulation) to infer a global picture of Early Toarcian environmental changes. M. Hermoso et al.: Record of Early Toarcian carbon cycle perturbations

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Conclusion